1. Field of the Invention
The present invention relates to an X-ray diagnostic apparatus, more specifically, relates to an X-ray diagnostic apparatus which is capable of observing high-definition still pictures (radiographic pictures) at real time and recording the radiographic picture data electronically.
2. Description of the Related Art
Conventionally, there have been known an X-ray diagnostic apparatus having a portable holding unit which is portable by means of casters provided to its main body, or a set-type holding unit which is used to be set on a ceiling.
This X-ray diagnostic apparatus, as shown in FIG. 1, has an X-ray control section 51 for controlling driving of an X-ray tube 52 which irradiates X rays to a patient, a film changer 54 for photographing an X-ray image in an X-ray film, an image intensifier 55 for converting the X-ray image into a visible light, and a camera head 58 for imaging the X-ray image based on the visible light from the image intensifier 55 through optical lenses 56 and 57.
In addition, this X-ray diagnostic apparatus has a camera control section 59 for controlling driving of the camera head 58, an image processing section 61 for performing a prescribed image process such as a video process on the X-ray image imaged by the camera head 58, a monitor unit 60 for displaying the X-ray image which was processed by the image processing section 61.
The X-ray tube 52 and image intensifier 55 are provided respectively to both end portions of a substantially C-shaped arm to face each other, for example, and when imaging, the X-ray tube 52 and image intensifier 55 are set so that a patient is located in the substantially middle portion therebetween (they sandwich the patient). When the X-ray tube 52 irradiates X rays to the patient, a formed X-ray image is captured by the image intensifier 55 to be converted into a visible light therein, and the visible light is imaged by the camera head so that the X-ray image is displayed on the monitor unit 60.
Here, in such an X-ray diagnostic apparatus, two kinds of imaging methods can be used. They are "fluoroscopy" in which a little amount of X rays are irradiated successively so that a moving picture is obtained, and "radiography" in which great amounts of X rays are irradiated at one shot so that a still picture is obtained. These "fluoroscopy" and "radiography" are selectively used according to cases.
In other words, in general, "fluoroscopy" is performed first. When the "fluoroscopy" is specified by an operator, the X-ray control section 51 shown in FIG. 1 controls driving of the X-ray tube 52 so that the X-ray tubes 52 irradiates a little amount of X rays successively. As a result, an X-ray image is formed through a patient. This X-ray image is converted into the X-ray image of a visible light by the image intensifier 55 to be imaged by the camera head 58. Then, the X-ray image is subject to the prescribed image process in the image processing section 61, and the X-ray image is supplied to the monitor unit 60 to be displayed thereon.
Since a little amount of X rays are irradiated in the "fluoroscopy" in order to reduce an amount of exposure, resolution of an obtained image (fluoroscopic image) and image quality such as S/N are not much satisfactorily, but since X rays are irradiated successively, a moving picture can be obtained. The operator chooses the proper time according to the moving picture and specifies "radiography" in desired timing.
When the "radiography" is specified by the operator, the X-ray control section 51 shown in FIG. 1 drives and controls the X-ray tube 52 so that the X-ray tube 52 irradiates great amounts of X rays at one shot. As a result, an X-ray image is formed through the patient. This X-ray image is printed in an X-ray film provided to the film changer 54.
Since this "radiography" is performed by the manner that X rays are irradiated at one shot, an obtained image (radiographic image) becomes a still picture, but great amounts of X rays are irradiated, so the radiographic image with excellent resolution and high quality such as S/N can be printed in the X-ray film. Then, a doctor makes diagnosis based on the radiographic image obtained by developing the X-ray film.
However, in the conventional X-ray diagnostic apparatus, there arose problems such that since radiographic image was obtained by printing it in the X-ray film, the radiographic image could not be checked until the X-ray film was developed, so in the cases where photography becomes failure and the picture of a desired affected part came out badly, it was necessary to radiograph again, and a wide space was required for X-ray films in which radiographic images were printed.
For this reason, it is desired to develop an X-ray diagnostic apparatus which can display also a radiographic image as well as a fluoroscopic image on the monitor unit, and can record the radiographic image in a data base electronically.
If such an X-ray diagnostic apparatus is developed, the following advantages can be obtained. A radiographic image can be checked on the monitor unit immediately without developing the radiographic image, and if the radiography becomes failure, the radiography can be performed again immediately. Further, since electronic storage becomes possible, reference becomes easy, and there is no difficulty in a storage place for the radiographic images.
However, since the conventional X-ray diagnostic apparatus was capable of imaging a moving picture by means of fluoroscopy, a so-called frame transfer, interline transfer or frame interline transfer CCD image sensor (FTCCD, ITCCD and FITCCD) which can be driven at high speed as the camera head 58 was provided. Therefore, when this CCD image sensor was used for the "radiography", resolution of an obtained radiographic image was deteriorated worse than that in an X-ray film, and thus a radiographic image which was useful for a diagnosis could not be obtained.
Namely, when such an CCD image sensor receives an X-ray image which was converted into a visible light by the image intensifier 55, all electric charges of the X-ray image are transferred into a light shielding area (memory area) at high speed in vertical interval time code, and they are read in order based on a driving clock (horizontal transfer). For this reason, the CCD image sensor is suitable for imaging a moving picture such as "fluoroscopy", but since a spatial sampling frequency is limited, it is limited to improve image quality, and thus the CCD image sensor could not be used for "radiography".
Accordingly, in the conventional X-ray diagnostic apparatus, at the time of "radiography", a high-definition radiographic image was obtained by printing the radiographic image in an X-ray film.
The number of pixels of the CCD image sensor should be increased in order to obtain a high-definition radiographic image in FTCCD, ITCCD and FITCCD, but if the number of pixels is increased without changing a size of the pixels (pixel size), there arises new problems that a whole CCD chip becomes large, and the number of chips obtained from one silicon wafer is reduced, so cost becomes high.
On the contrary, if a number of pixels is increased with the pixel size being small, the whole CCD chip becomes small, and thus the number of chips obtained from one silicon wafer is increased, but there arises new problem such that the number of saturation electrons which can be accumulated in one pixel is reduced, and excellent S/N characteristic cannot be obtained. This is because the S/N characteristic of the CCD image sensor is determined by quantum fluctuation of the number of electrons caused in pixels, and thus more satisfactory S/N characteristic can be obtained when the number of electrons is larger (pixel size is larger).
Accordingly, it is difficult to use FTCCD, ITCCD and FITCCD for the radiography with increasing the number of pixels.
Here, examples of the CCD image sensor existing except for FTCCD are full-frame CCD (FFTCCD), interline transfer CCD (ITCCD) and frame interline transfer CCD (FITCCD), but FFTCCD (full-frame CCD) is suitable for increasing the number of pixels from the viewpoints of the costs and S/N characteristic.
Namely, since FFTCCD has no light shielding area unlike another CCD image sensors and the pixels which are arranged in parallel in a vertical direction serve also as a vertical transfer path of electric charges, the pixel size is set to be larger, and further the whole chip size can be reduced, so FFTCCD is suitable from the viewpoints of the costs and S/N characteristic. For this reason, by means of FFTCCD, a high-definition radiographic image can be obtained.
However, since the FFTCCD has no light shielding area unlike FTCCD, an image signal cannot be outputted during accumulation of electric charges, and electric charges cannot be accumulated in each pixel during output of the image signal (a light cannot enter each pixel). For this reason, FFTCCD is necessary to be driven in at least intervals of an image output period, so if fluoroscopy is performed by using FFTCCD, successive irradiation of X rays is driven at interval of the image output period. As a result, a moving picture cannot be imaged satisfactorily. For this reason, when FFTCCD is provided in order to obtain a high-definition radiographic image, "fluoroscopy" is troubled.